The present invention relates to an electromechanical friction clutch and brake unit particularly for actuating and stopping a rotating element.
As is known, several technical sectors make use of electromagnetic friction clutch and brakes which are capable of actuating and stopping with due reliability a rotating element. In this respect, electromechanical friction clutch and brakes for sewing machines are known and widespread, which friction clutch and brakes ensure the immediate stoppage of the sewing machines.
The example of friction clutch and brake units for sewing machines is of considerable importance since it is precisely these machines which require brakes which must withstand difficult operating conditions. In fact, the said brakes must act on rotating elements which are usually able to reach speeds in the region of 10,000 revolutions per minute, and during a sewing operation the stopping and restarting phases are continually repeated. In practice, the brake must act continually, without showing any signs of failure. Moreover, in the case of these machines, the said brake must be substantially small in size. In particular, the currently known friction clutch and brake units for sewing machines are structured in accordance with various technical solutions. They have, nevertheless, some common components: a flywheel integral with a drive shaft, coupling disk integral with a driven shaft and selectively engageable, upon command, with the said flywheel and with stopping means. Also common is the provision of control and actuating means which are capable of causing the selective engagement of the said coupling disk against the fly wheel or against the stopping means. If the coupling disk is a normal, rigid disk, the control and actuating means displace the said coupling disk between the said flywheel and the said stopping means. If the coupling disk is an elastic disk sensitive to magnetic attraction, the control and actuating means deform this elastic disk so as to direct it either toward the flywheel or toward the stopping means. In other cases, the control and actuating means act in the manner of a pulsed brake or else act by means of magnetic attraction on a coupling disk formed mainly of magnetic particles.
In any case, the braking action is such that it reduces the number of revolutions of the coupling disk to approximately 250-300 revolutions per minute and then stops it immediately when a predetermined position is reached. With this process, a sewing machine performs no more than 3 to 5 stitches in between the stop command and the final immediate stoppage which occurs in such a way that the needle is situated at the top dead center or the bottom dead center with a very small margin of error.
The stopping means mentioned may be of various types, for example simple parts of a fixed housing surrounding the brake, against which the said coupling disk, or even an auxiliary electric motor unit which controls both the angular position and the axial position of the coupling disk, is arranged in a discontinuous manner.
In each case, all of the electromechanical positioning friction clutch and brake units currently known, in the particularly important sector of sewing machines, are provided with special friction elements which are inserted in the already mentioned means and are able to ensure an effective braking action and a degree of wear which at least lies within the limits of acceptability directly resulting from intense use of these machines and the need to stop them as little as possible for maintenance or overhauls. These friction elements are quite special since they must be inserted in elements rotating at very high speeds and consequently they must also be very light in weight.
In order to satisfy these difficult and contrasting requirements of long duration, maximum lightness and a suitable coefficient of friction, friction elements formed from a mixture of natural and synthetic products which have been suitably blended and treated have been universally used. In particular, friction elements made of cork have been used, on account of its lightness and low cost and in particular on account of the fact that it can be thoroughly impregnated with oil of suitable viscosity. This oil protects the cork or similar material from direct contact, and, because of its viscosity, is able to ensure an adequate coefficient of friction.
It has been shown that, for as long as the cork or similar material remains impregnated with oil, the brake possesses satisfactory characteristics, but as soon as the oil is used up and the braking action is performed as a result of direct contact of the cork, the latter becomes worn to the point of reducing the braking efficiency. It has not been possible to overcome this serious drawback by making the friction elements large in size, since cork is unable to withstand bending and twisting: it must be arranged in very thin layers. As a result of this fact, the quantity of oil stored inside the cork is also relatively limited.
In practice, electromagnetic friction clutch and brake units with different structures exist nowadays, but all of them are mainly based, at least recently, on the use of friction elements made of cork or a similar oil-impregnated mixture. Consequently, the friction clutch and brake units which exist nowadays are lightweight and in many respects satisfactory, and even last well with time, but are not such that the brakes themselves do not consitute a critical element in industrial sewing machines, with regard to their maintenance. In these circumstances, the technical task underlying the present invention is to devise an electromagnetic friction clutch and brake unit particularly of the type mentioned, which is able to overcome the drawbacks of the current state of the art, whilst ensuring perfect operation and minimum wear with time.